Processes for depositing multiple layers of different materials on a semiconducting substrate are fundamental to fabricating semiconductor devices. Typically deposited materials form conductive, semiconductive, or dielectric layers on the substrate. Processes for performing such deposition include chemical vapor deposition (CVD) and physical vapor deposition (PVD). In each of these processes, a source of molecular or atomic species provides molecular and/or atomic species which: travel towards an exposed surface of the substrate, deposit thereon, and build up to form a layer/film of deposited material. Molecular and/or atomic species from the source deposit on all surfaces exposed to the source of molecular or atomic species, i.e., the substrate and all surrounding surfaces.
Exclusion nngs (commonly known as shadow rings and clamp rings) are typically used to set an outside limit of deposition on a substrate. Such rings prevent deposition from occurring on the perimeter of the face of a semiconductor wafer underneath the rings and also assist in preventing deposition on the back side of the substrate.
A shadow ring typically overhangs or closely approaches the perimeter of the face of the substrate but does not rest on the surface of the substrate. In typical CVD processes a gas flow in the gap between the shadow ring and the substrate in a direction opposite the process gas flow purges the gap of process gas and prevents deposition on the face of the substrate adjacent its outer edge and also prevents deposition on the back side of the substrate. An example is shown in U.S. Pat. No. 5,328,722.
A clamp ring typically rests on the perimeter of the face of the substrate. Examples include those shown in U.S. Pat. No. 5,228,501, dated Jul. 20, 1993, to Tepman entitled, "Physical Vapor Deposition Clamping Mechanism and Heater/Cooler" and in European Patent Office publication EP-A 0598362 A1, dated May 25, 1994, entitled "Clamping Ring and Method and Apparatus for Using Same" and the references cited therein, to which reference is made for background descriptions of the structure and use of shadow rings and clamp rings in apparatus of the kind described. As described herein, the term "exclusion ring" is intended to include both shadow rings and clamp rings.
The repeated use of conventional shadow rings and clamp rings with multilayer film deposition processes produces a generally vertical film edge profile, where the removal of the vertical exclusion ring edge leaves the edges of underlying layers exposed. The exposure of the edges of reactive or conductive layers can circumvent the presence and purpose of an intermediate barrier layer which is intended to prevent undesirable chemical reactions or migration of material that might cause electrical contact between the two layers respectively above and below the barrier layer. When using such conventional rings, the presence of a barrier layer/film does not necessarily prevent interaction between the layers above and below the barrier layer at the exposed edge of the deposited film.
For example, in one conventional three layer deposition process a first metal layer of titanium is deposited over silicon, a second barrier layer of titanium nitride is deposited over the titanium, and finally a third metal layer of tungsten is deposited, because the titanium nitride barrier layer does not cover the exposed edge of the titanium layer. When the tungsten layer is deposited by a CVD process using tungsten hexafluoride (WF.sub.6), the WF.sub.6 has been observed to penetrate to the titanium layer at the perimeter of the film to cause a short circuit.
There is, therefore, a need for an improved deposition apparatus and method which can be used to reduce interaction between successively deposited layers at their exposed edges.